Nearly three decades later, as a visiting Kennedy conspiracy investigator from Britain, staying in the Texas ranch home of Marina Oswald Porter, he dared to tell Oswald's widow how he had celebrated the murder of her husband - the presumed assassin of his boyhood hero.
"Marina just told me how she understood what I was feeling that Sunday in 1963," Rudd remembers. "She said to me, 'John, that November they all told me that Lee did it. And I had to believe them. Now I know he was innocent'."
This month, John Rudd opens Europe's first annual 'Who Killed Kennedy?' conference in Liverpool.
From a computer at his electrical contracting firm, networking on the internet, within earshot of the Anfield Kop, he has persuaded 150 JFK investigators to gather on 27 and 28 July in the Edwardian grandeur of the Adelphi hotel for the first European conference, jointly mounted by the American Coalition on Political Assassinations (Copa) and a British group, Dallas '63.
Dr Charles Crenshaw, the surgeon who fought to save Kennedy's life in Trauma Room One at the Parkland Hospital, said last week in Texas: "I am coming over to tell this gathering in Liverpool that I saw that day in Dallas an entrance wound in the front of the president's head and an exit wound in the right rear of the head.
"In the autopsy report there is no sign of this wound. The wounds were changed in the forensic pathology report.
"I will describe the scenes in Parkland on 22 November, 1963. I will tell how the president's body was removed from our hospital by the Secret Service and how that action started all our problems in discovering the truth."
The Copa-Dallas '63 conference in Liverpool is the first chance for many conspiracy theorists to discuss the new American book Oswald Talked in which authors Ray and Mary La Fontaine claimed to reveal what Lee Oswald said to cellmate John Elrod in jail on the night after the killing of Kennedy.
The conclusion reached by the La Fontaines, that Oswald (the Patsy, as he called himself) believed he was working for the FBI in the weeks before the assassination, is now supported by Marina Oswald, according to Rudd.
Oswald Talked (Pelican Publishing, Gretna, Louisiana) claims that Lee Harvey Oswald carried a US government identity card which closely resembled the intelligence service's identity card found by the Russians in the personal effects of the shotdown U2 spy plane pilot Francis Gary Powers.
At his home in Oakfield Road, Liverpool, last week, John Rudd said: "The assassination is the most important piece of unfinished historical research in the world. It is not, and never was, a purely American problem.
"Like the quest for the truth about Dallas '63 our European conference is open to all-comers."
For conference details phone 0151-260 0654.Reuse content